Sex chromosome pairing in Drosophila melanogaster is restricted to sites called collochores located in the centrometric X heterochromatin and at the base of the short arm of the Y. It is not known whether collochores represent special functions or are regions of non-specific homology. Genetic studies have refined the map locations of the X collochores and have shown them to be functionally redundant, but have not identified the responsible sequence. P-element mediated transformation will be used to transform cloned DNA sequences onto pairing -deficient chromosomes which will then be tested for pairing ability. Transformed sequences will include the rDNA, a leading candidate for the natural pairing site, and a non-X unique sequence control to test for the efficacy of non-specific homology. Potentially important pairing parameters such as copy number and position will be manipulated. If pairing material is identified, smaller fragments will be tested to refine the identification. If these experiments fail, attempts to clone pairing material from genomic cosmid libraries will be made. Sequences adjacent to rDNA or to a heterochromatic break in a pairing region will be obtained and tested for pairing ability.